Josephson junctions are the weak connections between superconductors through which the Josephson effects are realized. Normally these junctions have a superconductor-insulator-superconductor configuration. However, the Josephson effect is observed in other configurations, such as a superconductor-normal metal-superconductor arrangement. Another configuration of a Josephson junction is a point contact in which a sharply pointed superconductor is brought into contact with a blunt superconductor. Thin-film microbridges can also form Josephson junctions. The simplest of these is a short narrow constriction on the order of a few micrometers in a superconducting film known as an Anderson-Dayem bridge. Other types of Josephson junctions are superconductor-semiconductor-superconductor, superconductor-oxide-normal metal-superconductor, as well as other artificial-barrier tunnel junctions. A Josephson junction can also be formed from the so-called SLUG junction consisting of a drop of lead-tin solder solidified around a niobium wire.
The DC current-voltage characteristics for different types of Josephson junctions may differ, but all show a zero-voltage supercurrent, and constant-voltage steps can be induced into the DC characteristics at voltages given by the following equation: EQU V=nhv/2e
where n is an integer, h is Planck's constant, v is the frequency of a voltage imposed across the barrier of the junction, and e is the magnitude of the charge of an electron.
Josephson junctions are used for extremely precise frequency-to-voltage conversion, and for measurement of absolute temperature. Josephson junctions, and instruments incorporating Josephson junctions, are also useful in other applications such as metrology at DC and microwave frequencies, magnetometry, detection and amplification of electromagnetic signals, and high-speed analog-to-digital converters and computers.
Josephson junctions can also be used as high frequency generators. When a Josephson junction is appropriately biased, its supercurrent oscillates at a well-defined fundamental frequency: EQU v=V/.phi..sub.0
where .phi..sub.0.apprxeq. 2.07mV/THz, and V is the time-averaged voltage across the junction. If the junction is biased such that V.gtoreq.i.sub.c R, where R is the junction resistance and i.sub.c is its critical current (a current value below which the junction is in the zero-resistance state), then the time-dependent voltage (output signal) is nearly sinusoidal. Under these conditions, the maximum power coupled to a matched load (where R equals R.sub.L) is P=i.sup.2.sub.c R/8. The theory and operation of Josephson junctions are described in the publications listed in the Appendix hereto, and incorporated herein by reference.
A single Josephson junction has been shown to couple 10nW to a neighboring junction at one THz, as described in the Robertazzi publication.sup.1 (see Appendix). However, there are two problems with single junction high frequency oscillators, a limited output power and output resistances that are too low to match typical load impedances, such as 50.OMEGA.. To overcome these drawbacks, a series array of identical Josephson junctions has been used as a high frequency oscillator. The impedance of the series array can be chosen to match the load impedance by choosing the appropriate number of junctions, such that the number of junctions N=R.sub.L /R. This arrangement is shown in the Tilley publication.sup.2 (see Appendix).
In a series array of Josephson junctions, if all the junctions can be made to oscillate with the same phase and frequency without applying external radiation or excitation, the junctions are considered mutually phase-locked and the array emits coherent radiation to a matched load at a power level N times that of a single junction. The output power of an array of N Josephson junctions under matched-load, phase-locked conditions is P.sub.N =NP.sub.1 =N i.sup.2.sub.c R/8 or expressed differently P.sub.N =i.sup.2.sub.c R.sub.L /8. Under these conditions the line width of emitted radiation for a series array of Josephson junctions is .DELTA.v=v=(41 MHz/.OMEGA.K)TR.sup.2 .sub.d /RN, where T is the operating temperature and R.sub.d is the differential resistance of a bare Josephson junction.
There are drawbacks to the use of a linear array of Josephson junctions. As described by Jain et al. and Hansen et al. (and other publications in the Appendix) phase locking in series arrays only occurs when the junctions have nearly identical characteristics, and only when certain types of loads are connected to the array. These particular loads are evidently necessary for inducing phase locking through feedback of the radiation emitted from the array. The expense and difficulty of fabricating identical Josephson junctions have resulted in only limited success in totally phase locking series arrays.
In order to compensate for differences in the Josephson junctions constituting a series array, the DC bias current has been symmetrically divided among the junctions. As described by the Wan et al. publication.sup.4 (see Appendix), such an arrangement has resulted in a maximum power of 1 .mu.W at a frequency of 350GHz into a 60.OMEGA. load. However, this is still less than the maximum expected power of 7.mu.W that could have been achieved with the same arrangement if all of the Josephson junctions were phase-locked.
Consequently, other techniques must be employed to achieve total phase locking thereby maximizing power output. Two-dimensional arrays of Josephson junctions have been used as described by the Clark publication.sup.5 (see Appendix) and the Mooij et al. publication.sup.6 (see Appendix). However, these publications are limited to small arrays of point contacts and microbridges respectively, coupled to resonant cavities, and requiring high frequency external excitation of the array. Further, as described by Jain et al. and Hansen et al..sup.3 (and other publications in Appendix) neither of these arrays achieves mutual phase-locking resulting in a coherent power output.
The aforementioned examples fall short of the ideal two-dimensional array which operates with mutual phase locking resulting in a coherent power output.